Determination of the molecular mechanism of specific cell-cell recognition remains an important biological research objective. This proposal seeks to address this question using the soybean- rhizobium infection process as a model. The mechanism by which rhizobia infect leguminous plants is a complex process involving functions of both plant and symbiont. An interesting feature of this process is the species specificity exhibited (i.e., only certain species of rhizobia will infect certain species of plants). The genetic mechanism by which this species specificity is expressed is a primary focus of this proposal. We have cloned DNA from Bradyrhizobium, the symbiont of soybean, and identified at least 7 distinct loci important to symbiotic performance. This proposal focuses on two of these loci thought to be involved in determination of host specificity. In other work, we have determined the mode of regulation of the nodLABCD gene cluster of B. japonicum which is thought to involve the nodD gene product and a specific, highly conserved DNA sequence 5' of the nodLABC genes. This sequence is apparently absent from the presumptive host specificity loci cloned. An important research objective will be to determine the mode of regulation of these genes. Lastly, we will undertake studies to determine the biochemical functions of the genes identified. The goal of this research is an understanding of the mechanism of host recognition in the infection of soybeans by B. japonicum.